Charging NIMH (and most modern battery chemistries) isn't done at constant voltage, but at constant current.
You'll need some fairly involved electronics to provide a constant current source and monitor the battery state.
Battery University has a pretty good description of the charging cycle of NIMH batteries.
The initial charge is at 1C (in your case, 175 mA). There are 3 different stop-checks (whichever is reached first):
- Temperature rise rate of the cell being charged.
- Voltage across the cell.
- NDV (negative delta voltage). Basically check for when the voltage rise of the battery stops rising at a certain rate.
At this point, the charger will charge the battery at significantly less current, repeating similar 3 checks and iteratively stepping down the current charge rate.
The linked page suggests that slow-charging a NIMH battery is difficult/impossible because it's very difficult to get reasonable readings for many of the time-dependent stop checks. You're basically relying on a time charge, which could excessively charge the NIMH cell and damage the cell.
The primary safety concern is excessively heating up the battery. You might risk rupturing the battery, though I don't think NIMH are susceptible to catching fire as some other cell chemistries are. Most likely the battery casing is designed to rupture in a safe manner, but no guarantees.
It's unlikely you'll do anything which would be considered a significant safety concern, just use your common sense. There aren't any high voltages involved, and the power outputs are relatively low.
You're not actually charging your laptop battery with the solar system battery. (At least, you shouldn't be.) Your laptop includes charging circuits to make sure your battery gets the voltages and currents to charge it properly. So long as those circuits are fed with voltage and current within their operating range, your laptop shouldn't care what the exact nature of the source is. AC line and switching converter, charged battery, discharged battery, all that's going to matter are voltage and current capacity.
Of course, there are a lot of details to "voltage and current capacity" that might bite you, this is a gross oversimplification, your mileage may vary, professional driver on a closed course, American Express cards not accepted.
Best Answer
@dike are you referring to that the AC units are directly connected to the solar panels and do not have electrical plugs on them and then connected to the panels? If so then research solar panel power adapters. And by time the correct term is trickle charge.